As one of the famous "four huai medicines", Rehmannia glutinosa has a longstanding usage history. This study aimed to determine how variations in soil bulk density impact the respiration metabolism of R. glutinosa root tubers and, consequently, the quality of its medicinal components. We evaluated the enzymatic activities and key metabolic compounds of R. glutinosa across a range of soil bulk densities, which included: pyruvate kinase (PK), pyruvate decarboxylase (PDC), alcohol dehydrogenase (ADH), lactate dehydrogenase (LDH), isocitrate dehydrogenase (IDH), succinate dehydrogenase (SDH), malate dehydrogenase (MDH), and the content of pyruvate, ethanol, lactic acid, succinic acid, malic acid, and quality indicators. High soil bulk density increased the activities of PK, PDC, ADH, and LDH, while decreasing SDH, MDH, and IDH in the roots. This shift in enzymatic activities corresponded to elevated levels of ethanol and lactic acid, and lowered levels of succinic and malic acids. In the additionally, the growth and development of aboveground parts of R. glutinosa were seriously hindered under high bulk weight treatment, and the number of root tubers decreased, root coarser and root length shortened, and the contents of catalpa alcohol and mullein were significantly reduced. Our findings suggest an optimal soil bulk density for R. glutinosa growth to be between 1.2 g/cm3 and 1.4 g/cm3. Based on changes in respiratory metabolism and medicinal quality indicators of R. glutinosa under different soil bulk densities, our findings suggest the optimal soil bulk density for R. glutinosa growth is 1.2 g/cm3 to 1.4 g/cm3. Excessively high soil bulk density will affect the growth and quality formation of R. glutinosa by influencing its metabolism. Recognizing this optimal range is crucial for maximizing the medicinal quality of R. glutinosa, offering valuable insights for cultivation practices.